The invention relates to the measurement of intraocular pressure and, in particular, to methods and apparati for measuring intraocular pressure using applanation tonometry.
The measurement of intraocular pressure (IOP) is essential to the diagnosis and management of glaucoma, a major cause of blindness in the United States and around the world. Although direct measurement of intraocular pressure can be obtained by inserting a pressure sensitive probe into the eye, clinical methods must rely on indirect methods of obtaining intraocular pressure. There are two popular methods for obtaining these indirect measurements. In a first method, the eye is indented using a tonometer, popularized by Schiotz, wherein gram weights are placed on a central post that exerts pressure on a globe relative to a curved plate that covers an anesthetized cornea. This method has several disadvantages, which include the requirement that the patient remain in a supine position in addition to errors in measurement related to scleral rigidity. Also, the Schiotz tonometer requires a normogram to interpret the measured pressure.
The second method for obtaining an indirect measurement of intraocular pressure is the applanation technique, wherein a portion of the cornea is flattened by a mechanical device. In applanation techniques, the force required to produce a flattening of the cornea is related to intraocular pressure, so the intraocular pressure can be determined indirectly by measuring the flattening of the cornea and the pressure required to produce that flattening. Goldmann determined the exact area required such that one gram of force is equivalent to one mm Hg of intraocular pressure. McKay and Marg developed an electronic tonometer based on differential applanation between a central post and a surrounding annulus. This principle is utilized by the TONO-PEN(trademark) electronic tonometer, patented by Feldon et al. in U.S. Pat. No. 4,747,296. A variant of the applanation methodology requires no direct contact with the patient""s eye. This xe2x80x9cair puffxe2x80x9d technique involves directing a calibrated packet of pressurized air onto the corneal surface, which causes corneal flattening. This corneal flattening is then measured indirectly by measuring the deflection of light reflected from the corneal surface.
Of the various types of tonometers available for clinical use, the Goldmann applanation tonometer is considered the xe2x80x9cgold standard.xe2x80x9d However, there are several shortcomings to this technique. First, the device, as originally designed, is not portable, but is attached to a slit lamp. This issue has been addressed by the Perkins and Kowa tonometers that incorporate a Goldmann-type tonometer in a portable design. Second, these tonometers are manual devices that rely upon a highly trained observer to obtain reliable results. Third, the prolonged amount of time required to position the patient as well as poor patient tolerance make Goldmann tonometry inefficient and sometimes impossible to perform. Finally, a Goldmann tonometer touches the eye with a non-disposable device that is difficult to sterilize between uses. This increases the likelihood of transmitting infectious diseases or causing chemical damage to the cornea from residual antiseptic coming into contact with the patient""s eye.
An applanation tonometer and method for measuring intraocular pressure are described herein. The invention allows for accurate measurement of intraocular pressure while addressing some of the deficiencies of existing tonometers.
In one embodiment, an applanation tonometer for flattening the cornea of an eye is provided, wherein the applanator comprises a fiberoptics array; a force transducer for measuring forces applied by the applanator; an image transducer for obtaining data regarding an image of the applanated eye; and a processing circuit configured to calculate an intraocular pressure of the eye from at least a measured force and corresponding measured geometrical property. In aspects of the preferred embodiment, the geometrical property of the image may comprise an area, diameter, or major and minor axes of the cornea of the applanated eye. The tonometer may further comprise a light source for illuminating the eye, and a lens system adapted to focus an image of the applanated portion of the eye from the applanator to the image sensor. Advantageously, an embodiment of the tonometer calculates an intraocular pressure using a plurality of corresponding forces and geometrical properties of the applanation image. In another aspect of a preferred embodiment, the tonometer is provided with a disposable tip that covers the applanator, thereby providing a replaceable, sterile interface between the tonometer""s applanator and a patient""s eye.
In another embodiment, a method of determining an intraocular pressure of an eye comprises applanating a portion of an eye of varying degree over time, acquiring data relating to the forces required to applanate the eye and the geometry of applanated portions of the eye, determining a slope of a line defined by these of data points, and calculating the intraocular pressure using this slope and a predetermined relationship. In a preferred embodiment, the data acquisition is synchronous.
In another preferred embodiment, a disposable tip for use with an applanation tonometer comprises a light-transmitting applanating region adapted to fit over an applanator of a tonometer, the applanating region having a surface adapted to optically couple with an applanator and another surface adapted to contact an eye, and an attachment mechanism for detachably attaching the tip to a tonometer. In an aspect of this preferred embodiment, the surface of the tip used to contact the eye has a matte finish, and the applanating surface of the tonometer""s applanator has a matching flat, polished surface. Preferably, the tip is sterilized, and may further be packaged within a sterile environment. Advantageously, the tip may further incorporate a key fitting for attaching to the tonometer.
The tonometer embodiments described herein apply broadly and may take any one of various forms, such as a portable, hand-held device or a device uniquely designed to be used in conjunction with a slit-lamp.